Generally, the hoist and traction apparatus is provided with an overload prevention device which allows the load sheave to slip when subjected to an overload so that a load is restrained from being further hoisted or hauled in order to prevent the load-hoisting or -traction in the overload state exceeding a preset load.
The overload prevention device, as disclosed in, for example, the Japanese Patent Laid-Open Gazette No. Sho 51-106950, is provided at an intermediate shaft of a reduction gear mechanism provided between the driving shaft of the motor and the load sheave, in other words, between a second reduction gear and the intermediate shaft for supporting the reduction gear or is provided between the driving shaft and a first gear shaft of the reduction gear mechanism, as disclosed in, for example, the Japanese Patent Publication Gazette No. Sho 63-3831.
However, in the case where the overload prevention device is provided at the intermediate shaft, an axial length thereof is larger and a gear casing is enlarged to that extent to thereby create the problem in that the apparatus is large-sized as a whole. Usually, at the driving shaft is provided an electromagnetic brake, or, in some cases, a mechanical brake, for holding the hoist position when the motor stops. Since the overload prevention device is provided at the load side of the brake, that is, at the load sheave side, in the case where the hoist and lowering operation are in severe repetition, the so-called inching operations are performed, near a slip load set by the overload prevention device, in other words, preset set value of a transmitting torque, a friction plate of the overload prevention device is heated. As the result, in a case where the friction plate is changed in friction property of a friction member, extremely rarely, even when subjected to no external force, the overload prevention device slips to lower the load, thereby creating a problem because the load cannot be held at the predetermined position.
Also, in the case where the overload prevention device is provided between the driving shaft and the first gear shaft as disclosed in the Japanese Patent Publication Gazette No. Sho 63-3831, a problem will be created as follows:
The hoist and traction apparatus disclosed in the above-mentioned Gazette is provided with an overload prevention device in such a manner that the first gear shaft projects from a side plate at the motor side toward a motor room and one axial end of the driving shaft is fitted to be rotatably relative to the axial end of the projecting shaft portion so as to be supportingly connected with each other and then the overload prevention device is provided at a juncture of the axial end portions. Therefore, the latter problem of the conventional example is solved, but a total axial length of the first gear shaft and driving shaft becomes larger in comparison with the case where the over load prevention device is not provided, and the motor casing must be made larger so that the problem still remains in that the apparatus is as a whole of a large size. Moreover, the one axial end of the driving shaft is positioned in the motor room and supportingly filled into axial end of the first gear shaft, whereby it is difficult to align the first gear shaft and driving shaft. Also, the driving shaft, which is relative-rotatably supported to the axial end of the first gear end, causes deflection, thereby creating another problem in that wear is generated or noises are produced.